Distributed re-encryption apparatus, cryptographic system, distributed re-encryption method, and distributed re-encryption program

Abstract

A distributed re-encryption apparatus comprises: a distributed re-encryption key storage part that stores shares obtained by secret-sharing a re-encryption key that re-encrypts a ciphertext into one encrypted with a different encryption key without decrypting the ciphertext; and a distributed re-encryption part that re-encrypts using the shares of the re-encryption key the ciphertext into one encrypted with a different encryption key without decrypting the ciphertext.

What is claimed is: 
     
         1 . A distributed re-encryption apparatus comprising:
 a distributed re-encryption key storage part that stores shares obtained by secret-sharing a re-encryption key that re-encrypts a ciphertext into one encrypted with a different encryption key without decrypting the ciphertext; and   a distributed re-encryption part that re-encrypts using the shares of the re-encryption key the ciphertext into one encrypted with a different encryption key without decrypting the ciphertext.   
     
     
         2 . The distributed re-encryption apparatus according to  claim 1 , wherein the ciphertext is a result of a homomorphic operation between ciphertexts encrypted with different keys. 
     
     
         3 . The distributed re-encryption apparatus according to  claim 1 , wherein the re-encryption key is configured by combining part of a decryption key for the ciphertext encrypted with the different encryption key and a decryption key for the ciphertext. 
     
     
         4 . The distributed re-encryption apparatus according to  claim 1 , wherein the ciphertext encrypted with the different encryption key is obtained by decrypting what are computed as secret shares. 
     
     
         5 . A cryptographic system having:
 a plurality of the distributed re-encryption apparatuses according to  claim 1 ;   a plurality of key generation apparatuses, each of which comprising an encryption key generation part that generates an encryption key in multi-key fully homomorphic encryption, a decryption key generation part that generates a decryption key in the multi-key fully homomorphic encryption, an evaluation key generation part that generates an evaluation key in the multi-key fully homomorphic encryption, a re-encryption key generation part that generates the re-encryption key, an encryption key storage part that stores the encryption key, and a decryption key storage part that stores the decryption key;   a plurality of encryption apparatuses, each of which comprising a ciphertext generation part that generates a ciphertext using the encryption key in the multi-key fully homomorphic encryption;   an encrypted data operation apparatus comprising a ciphertext storage part that stores a ciphertext, an evaluation key storage part that stores an evaluation key used for an operation between the ciphertexts, and an operation part that performs a homomorphic operation on the ciphertexts; and   a decryption apparatus that decrypts the ciphertext encrypted with the different encryption key,   connected to each other by a network.   
     
     
         6 . The cryptographic system according to  claim 5 , wherein
 the re-encryption key is configured by combining part of a decryption key for the ciphertext encrypted with the different encryption key and a decryption key for the ciphertext.   
     
     
         7 . The cryptographic system according to  claim 5 , wherein the ciphertext encrypted with the different encryption key is obtained by decrypting what are computed as secret shares. 
     
     
         8 . A distributed re-encryption method including:
 acquiring shares obtained by secret-sharing a re-encryption key that re-encrypts a ciphertext into one encrypted with a different encryption key without decrypting the ciphertext; and   re-encrypting using the shares of the re-encryption key the ciphertext into one encrypted with a different encryption key without decrypting the ciphertext.   
     
     
         9 . The distributed re-encryption method according to  claim 8 , wherein the ciphertext is a result of a homomorphic operation between ciphertexts encrypted with different keys. 
     
     
         10 . The distributed re-encryption method according to  claim 8 , wherein
 the re-encryption key is configured by combining part of a decryption key for the ciphertext encrypted with the different encryption key and a decryption key for the ciphertext.   
     
     
         11 . The distributed re-encryption method according to  claim 8 , wherein
 the ciphertext encrypted with the different encryption key is obtained by decrypting what are computed as secret shares.   
     
     
         12 . A non-transitory computer readable medium storing a distributed re-encryption program causing an information processing apparatus to execute:
 acquiring shares obtained by secret-sharing a re-encryption key that re-encrypts a ciphertext into one encrypted with a different encryption key without decrypting the ciphertext; and   re-encrypting using the shares of the re-encryption key the ciphertext into one encrypted with a different encryption key without decrypting the ciphertext.   
     
     
         13 . The non-transitory computer readable medium storing the distributed re-encryption program according to  claim 12  wherein the ciphertext is a result of a homomorphic operation between ciphertexts encrypted with different keys. 
     
     
         14 . The non-transitory computer readable medium storing the distributed re-encryption program according to  claim 12  causing an information processing apparatus to execute obtaining the ciphertext encrypted with the different encryption key by decrypting what are computed as secret shares. 
     
     
         15 . The non-transitory computer readable medium storing the distributed re-encryption program according to  claim 12  wherein
 the re-encryption key is configured by combining part of a decryption key for the ciphertext encrypted with the different encryption key and a decryption key for the ciphertext.